Relief Valve Discharge to Blowdown Tank Vent

[CountOlaf]

Dear Colleagues:

I've got a system at a coal-fired power plant where we've put in a pressure relief valve off of a water loop which is used for automatic cleaning of the furnace (like water cannons). The water source is condensate water at relatively low temp (100 deg F) and high pressure (say 600 psig). The relief valve is set at about 1,000 psig and is there just to prevent an overpressurization in this subsystem if certain valves are inadvertainly closed and higher pressures occur. We've called for routing of the relief valve discharge to the unit's blowdown sump at the ground level of the plant. The relief valve discharge line is a 1" size.

The unit's boiler blowdown tank which dumps to this same blowdown sump (and of course receives effluent from the main boiler drum and other areas) has a 24" vent line which runs all the way up and out of the plant thru the roof. Temps in this vent line and in the tank itself are around 190 deg F (and presumably it's at atmospheric pressure).

Rather than run the 1" relief discharge line all the way down to the sump, someone suggested connecting to this 24" blowdown tank vent line since it's nearby the relief valve and of course the discharge will get to the same ultimate place.

It seems logical and cost effective to do so, but I wonder if it's allowed? I don't see other blowdown lines connected to this vent line (could be just because the blowdown lines (and hence the connected vent line) would have to be constructed of better material to handle the higher pressures invloved), or maybe it's just not allowed in the same way you wouldn't want to connect a new sanitary line to an existing vent line in a domestic plumbing system.

Anyway, your thoughts on this matter are greatly appreciated.

 

[rmw]

I'd do it. If it is as you describe, that 24" line will never know that that one inch line is dumping into it. If the safety valve ever reieved at a time when the blowdown tank vent had a high flow, you might get some reverse flow which would create a shower in the vicinity of the vent line, but I would think that the possibility of that was remote.

rmw

 

[MikeHalloran]

But if you so hide its discharge, you may never know that the relief valve has opened. Concealing a fault indication does not sound like a good idea.





Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

I think it wouldn't be hard to know if a 1" relief valve relieved at 1000 psi. It wouldn't be quiet about it.

rmw

 

[MikeHalloran]

Agreed, it would be noisy, but in a powerplant, you still might not hear it.



Mike Halloran
Pembroke Pines, FL, USA

 

[CountOlaf]

Thanks for the insight so far guys....

Regarding the "concealing of the fault" debate, in the more costly option, we would have discharged the pipe through the metal cover plating into the sump, so we would not have had a readily visible indication of discharge anyhow.

I guess my only other misgiving about the vent line discharge now is that if there is an event, it wil blast 1000 psig water into the opposite inner side of this 24" vent pipe/stack. I suppose they could do some sort of turned down elbow arrangement (as opposed to trying to add a wear plate).

 

[rmw]

Mike, I thought about the power plant ambient noise as I posted that. So much for that idea.

CountOlaf, I figured you were putting it into a non visible sump or tank anyway.

The pressure of the water downstream is not 1000 psi, because of the pressure drop across the valve. The velocity at which the water arrives at the vent stack then depends on the size of the line you install downstream of the relief valve. If the line size is small, then you either impact the opposite side of your stack or the back side of the elbow that turns the flow down.

rmw

 

[EdStainless]

You might want to run this 1" line in stainless, at least the last few feet, the elbow and the down leg. Otherwise you could erode through it quickly and then your vent would be in danger.

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